Sunset on the African savanna, a long, long time ago, and a clan of human ancestors are engaged in a little after-dinner chat. They’re celebrating their good fortune; how often do you stumble across a fresh antelope carcass right next to a patch of ripe berries? A child asks how the adults got the meat off the antelope, so they demonstrate the use of stone blades, explaining how it’s all in the wrist. The conversation is animated, but if you could eavesdrop on it, all you’d hear would be occasional grunts and squeals. The hominids aren’t talking; they’re communicating by means of swift, complex hand gestures--that is, in sign language.

To many paleoanthropologists, that scenario would sound far- fetched. Ever since researchers began to wonder how our ancestors evolved language, they have assumed that language equals speech. But in a recently published book, an anthropologist and two linguists whose specialty is the modern sign language of the deaf challenge this assumption. Language, they claim, did not emerge suddenly in modern humans, as many researchers believe, within the past 50,000 years or so. Instead it evolved gradually, beginning with our earliest hominid and even our primate ancestors. And at first it was a language of gestures rather than words.

There is no direct evidence, of course, that early hominids had any language at all. But anthropologist David Armstrong of Gallaudet University--a Washington, D.C., school for the deaf--and his colleagues have one piece of indirect evidence for their speculation. The size of hominid brains increased dramatically long before Homo sapiens emerged, from 24 cubic inches or so 4 million years ago to an average of 60 cubic inches among Homo erectus, the people on that African savanna. (Human brains range in size from around 60 to 100 cubic inches.) The bigger brain was not simply running a bigger body; the brain-to-body ratio exploded as well, in a way that has not been seen in any other animal lineage. “The question is, what were the early hominids doing with those brains?” says Armstrong.

Figuring out how to use tools is one possibility. But chimpanzees today, with brains a third the size of ours, use sticks as tools, and the chipped stones of early hominids weren’t much more complicated. Language is a more sensible explanation for swelling brains, says Armstrong; it would have required far more mental capacity, and it would have offered clear advantages to a troop of hunter-gatherers. Yet early hominid language, if it existed, probably couldn’t have been spoken language--because early hominids probably couldn’t speak. Fossils suggest that their throats were like those of chimps: the pharynx, the chamber just above the vocal cords where sounds can be modulated, was too small to make the complex noises necessary for speech.

The notion that hominids didn’t need speech for language--that they used sign language instead--has come up before, but it has never been taken very seriously, in part because sign language itself hasn’t been. Linguists traditionally considered sign language a crude aping of spoken English. That attitude began to change in the 1960s, though, thanks to the efforts of people like linguist William Stokoe, also of Gallaudet, who showed that signing is as linguistically complex as speech. More-recent research has demonstrated that signing draws on many of the same parts of the brain that speech does, and that deaf children learn sign language much as children who can hear learn speech, sign-babbling away. “When I first proposed that signing was a language itself and not just a way of representing spoken English, the idea didn’t go down well,” says Stokoe. “But the changes since then have been just astounding.”

The time is thus ripe for a new book, Gesture and the Nature of Language, in which Stokoe, Armstrong, and Sherman Wilcox of the University of New Mexico argue that signing is not only a fully equivalent language but may have been the first language of all. In their scenario, the seeds of sign language were planted in our chimplike ancestors. Primates in general have highly developed vision, and they have specialized bundles of neurons that respond strongly to the sight of moving arms. “The primate nervous system may have already been tuned to this sort of stimulus,” says Armstrong. In addition, chimpanzees can be taught a crude sort of sign language, suggesting that the brain of our common ancestors was beginning to be capable of language.

But chimps are held back from a full-blown sign language by two limitations: their small brains and their hands. “Chimpanzees are knuckle walkers and have relatively long fingers and short thumbs, so the thumb isn’t readily opposable as it is in humans,” explains Armstrong. “One of the things you notice when you watch films of chimps signing, if you know much about sign language, is that they simply can’t make some of the basic hand shapes.” When our ancestors became bipeds, however, their hands quickly took their modern form. That allowed them to wield their primitive tools more skillfully--and it also allowed them to make complex hand gestures. Long before we had a throat fully capable of speech, we had hands fully capable of sign language.

With signing rather than speech as a starting point, one of the central mysteries in the origin of language--the question of how people first came to combine words into sentences--becomes a lot less mysterious. “The usual idea is that hominids worked up this tremendous list of words and they finally got so many they had to classify them into categories like nouns and verbs,” says Stokoe. “Well, who told them how to do that? I don’t know how many vocal signs you’d have to get before the idea would dawn on someone that some of them represented actions and some represented actors and you could put them together.”

In sign language, the gap between word and sentence can be bridged naturally in a single sign. The sign for grab in American Sign Language, for example, is one hand closing around an outstretched finger on the other hand. But this gesture can also mean he grabbed it. The moving hand embodies the subject, the movement is the action, and the other hand is the direct object. “If an ancient hominid saw a leopard jump on a gazelle,” says Stokoe, “he might have gone and made a gesture to another hominid that was a motion of the hand like the front paws of the leopard and grabbed his other hand. That doesn’t say leopard, but the leopard jumped on something. You get the relationship of the thing to what it does in one unit.” Other aspects of grammar could have emerged in a similarly natural way; facial expressions, for example, are natural adverbs, modifying the actions that are signed.

Once signing had permitted the invention of sentences, though, our ancestors eventually switched to speech. “For an animal that uses tools and probably moves around and has to carry things, it’s going to make a lot more sense that the primary communication be vocal,” says Armstrong. But perhaps that primordial sign language never really died out. “People who have no contact with deaf people use a whole repertoire of manual gestures, and some are identical to the signs in sign language,” Armstrong says. “We all have some common stock of signs.”

While humans may have shifted from signs to speech long ago, Stokoe, Armstrong, and Wilcox (none of whom are deaf) don’t want people to think that they consider modern sign language primitive--just the opposite. “One of the motivations of my work,” says Stokoe, “is to let people know that deaf people are just as bright and able to handle abstract thought as hearing people, given equal education. Prejudice against them because they don’t speak leads to all sorts of bad judgments about deaf people. The fact that we all probably owe a great deal to the era when language began in a gestural state might change some of that.”